Fertilizers are, as a rule, not fully utilized by the crop with much of the fertilizer material being lost to the soil environment and the atmosphere. This occurrence is encountered practically with all fertilizers and is particularly pronounced with nitrogen-based fertilizers. The main reason for this deficiency is the very water solubility of the fertilizer a result of which undesired dissipation of the fertilizer by drainage, its consumption or decomposition by microbes and losses to the atmosphere in form of NH.sub.3, N.sub.2 O, NO and NO.sub.2, compete with the root uptake thereof.
It has been suggested to overcome this problem by means of chemically prepared slow-release fertilizers such as, for example, ureaform, isobutylidene diurea, oxamide and others, in which the desired nutrient element such as nitrogen, is present in form of a compound of reduced water solubility. This method, however, has, among others, the disadvantage that it requires expensive raw materials and involves high manufacturing costs with the result that the price of the product may be prohibitive. Thus, for instance, in case of nitrogen-based fertilizers, the use of nitrogen compounds of reduced water solubility is at least three times as costly as the use of regular, water soluble nitrogen fertilizers. Moreover, due to the so-called tailing effect, i.e. too slow decomposition upon application to the soil, the efficiency of chemically prepared fertilizers of reduced water solubility is often still unsatisfactory.
It is also known to control the dissolution rate of fertilizers by means of a physical barrier provided by coating of individual fertilizer particles with a composition of restricted water permeability or encapsulation of a cluster of fertilizer particles in matrices of such compositions, and this type of slow-release fertilizer products is capturing a steadily increasing market share. In the following, the terms "encapsulation" and "encapsulated" will be used in relation to slow-release particulate fertilizer of this kind regardless of whether each particle thereof is coated individually or whether clusters of such particles are encapsulated within matrices.
Encapsulated slow-release fertilizers include two main types:
(a) a first type characterized by encapsulation in a thin, high molecular polymer membrane; and PA1 (b) a second type characterized by encapsulation in a thick coating of a relatively inexpensive raw materials, e.g., sulfur, possibly in combination with wax and a powderous mineral. PA1 (i) While at temperatures above 150.degree. C. elementary sulfur is amorphous or polymeric, which is the preferred physical state for coating, upon cooling it reverts to crystalline form in which it has a high surface tension, whereby its capacity of adhering thereto is reduced. As a result, coating defects may occur which lead to deficiencies such as low abrasion resistance; peeling off during storage, handling and soil application; and others. PA1 (ii) Due to defects in the encapsulating coat, a large proportion of the fertilizer of the order of 30-50%, may be released already within a few days in water or in moist soil by a so-called burst effect, which may cause phytotoxicity problems and losses of nitrogen. PA1 (iii) The encapsulating coat on parts of the fertilizer particles may be too thick, giving rise to a so-called tailing or lock-off effect, i.e. to a slow down of the fertilizer release to a point where common field crops cannot utilize it fully during the growth season and a significant proportion of the fertilizer thus fails to reach the crops and is lost. PA1 (iv) Encapsulating coats with a relatively high content of free sulfur in the fertilizer product, may cause an undesired acidification of the growth beds.
The release characteristics and mechanical properties such as attrition resistance, crushing strength, etc. of slow-release fertilizers of the first type in which the fertilizer is encapsulated in a thin polymer membrane, are superior to the release characteristics and mechanical properties of those of the second type in which the fertilizer is encapsulated in a thick coat. However, polymer membrane encapsulation requires expensive raw materials and the production technologies are difficult, complex and quite costly. Furthermore, in some of the polymer-encapsulated slow-release fertilizers, the degradation of the coating is slow and may occasionally take longer time than the time required for the release of all the amounts of nutrients. Where this happens residual polymer accumulates in the soil.
The thick-encapsulated slow-release fertilizers are usually prepared by coating with solidifiable, water-insoluble melts such as sulfur melts. Sulfur is attractive due to its low cost and also due to the fact that it is in itself a plant nutrient. However, conventional sulfur encapsulation has, among others, the following deficiencies:
The inferior mechanical properties of known slow-release fertilizers with sulfur encapsulation may be improved by the addition of wax, if desired together with a conditioning agent such as clay. However, both sulfur and waxes undergo microbial degradation in biological active soils and therefore in many cases the addition of microbiocides is required in order to prevent a fast release.
U.S. Pat. No. 3,295,950 describes sulfur coated slow-release fertilizers with which in the coating include 2 to 10% by weight of a polysulfide plasticizer, whereby the quality of the coating is allegedly improved. U.S. Pat. No. 4,636,242 discloses sulfur coated slow-release fertilizers in which the coating includes about 0.5% to 2% by weight of the coating of a symmetrical dialkyl polysulfide, and it is alleged that this imparts plasticity to the crystalline sulfur whereby the coating quality is improved. It is further mentioned that the plasticizer also reduces the surface tension and viscosity of the sulfur coating.
It is the object of the present invention to provide new slow-release fertilizer products with a new type of sulfur based, release governing composition of limited water permeability.